OnJava8-Examples/net/mindview/atunit/ClassNameFinder.java

//: net/mindview/atunit/ClassNameFinder.java
package net.mindview.atunit;
import java.io.*;
import java.util.*;
import net.mindview.util.*;
import static net.mindview.util.Print.*;

public class ClassNameFinder {
  public static String thisClass(byte[] classBytes) {
    Map<Integer,Integer> offsetTable = new HashMap<>();
    Map<Integer,String> classNameTable = new HashMap<>();
    try {
      DataInputStream data = new DataInputStream(
        new ByteArrayInputStream(classBytes));
      int magic = data.readInt();  // 0xcafebabe
      int minorVersion = data.readShort();
      int majorVersion = data.readShort();
      int constant_pool_count = data.readShort();
      int[] constant_pool = new int[constant_pool_count];
      for(int i = 1; i < constant_pool_count; i++) {
        int tag = data.read();
        int tableSize;
        switch(tag) {
          case 1: // UTF
            int length = data.readShort();
            char[] bytes = new char[length];
            for(int k = 0; k < bytes.length; k++)
              bytes[k] = (char)data.read();
            String className = new String(bytes);
            classNameTable.put(i, className);
            break;
          case 5: // LONG
          case 6: // DOUBLE
            data.readLong(); // discard 8 bytes
            i++; // Special skip necessary
            break;
          case 7: // CLASS
            int offset = data.readShort();
            offsetTable.put(i, offset);
            break;
          case 8: // STRING
            data.readShort(); // discard 2 bytes
            break;
          case 3:  // INTEGER
          case 4:  // FLOAT
          case 9:  // FIELD_REF
          case 10: // METHOD_REF
          case 11: // INTERFACE_METHOD_REF
          case 12: // NAME_AND_TYPE
            data.readInt(); // discard 4 bytes;
            break;
          default:
            throw new RuntimeException("Bad tag " + tag);
        }
      }
      short access_flags = data.readShort();
      int this_class = data.readShort();
      int super_class = data.readShort();
      return classNameTable.get(
        offsetTable.get(this_class)).replace('/', '.');
    } catch(IOException | RuntimeException e) {
      throw new RuntimeException(e);
    }
  }
  // Demonstration:
  public static void main(String[] args) throws Exception {
    if(args.length > 0) {
      for(String arg : args)
        print(thisClass(BinaryFile.read(new File(arg))));
    } else
      // Walk the entire tree:
      for(File klass : Directory.walk(".", ".*\\.class"))
        print(thisClass(BinaryFile.read(klass)));
  }
} ///:~